As Lilly says, when clicking on the dustbin icon the post will be soft-deleted (that is, only visible to admins and mods[1]). The delete removed posts after time is how long a user has the option to ‘undelete’ their post and reverse their decision before it becomes visible to only admin/mods.
To hard-delete a post you would need to enable the hidden site setting can_permanently_delete, which would allow an admin to permanently delete that post from the database with no means to restore it.
More info can be found in:
Introducing permanently delete post functionality
Enable setting to allow admins to permanently delete data
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and TL4 if
tl4 delete posts and topicsis enabled ↩︎